On 12/08/21 08:49, Marc Zyngier wrote:

On Tue, 29 Jun 2021 13:49:59 +0100,
Valentin Schneider [off-list ref] wrote:

quoted

+void eoi_irq(struct irq_desc *desc)
+{
+	desc->irq_data.chip->irq_eoi(&desc->irq_data);
+
+	if (desc->irq_data.chip->flags & IRQCHIP_AUTOMASKS_FLOW)
+		irq_state_clr_flow_masked(desc);

I just realised that this has a good chance to result in a mess with
KVM, and specially the way we let the vGIC deactivate an interrupt
directly from the guest, without any SW intervention (the magic HW bit
in the LRs).

I didn't think to consider those. It can't ever be simple, can it...

With this, interrupts routed to a guest (such as the timers) will
always have the IRQD_IRQ_FLOW_MASKED flag set, which will never be
cleared.

I wonder whether this have a chance to interact badly with
mask/unmask, or with the rest of the flow...

Isn't it the other way around? That is, eoi_irq() will clear
IRQD_IRQ_FLOW_MASKED regardless of what happens within chip->irq_eoi(),
so we would end up with !IRQD_IRQ_FLOW_MASKED even if the (physical) IRQ
remains Active (irqd_is_forwarded_to_vcpu() case).

This does not entirely match reality (if the IRQ is still Active then it is
still "flow-masked"), but AFAICT this doesn't impact our handling of
forwarded IRQs: IRQD_IRQ_FLOW_MASKED is only really relevant from ack_irq()
to eoi_irq(), and deactivation-from-the-guest (propagated via LR.HW=1)
happens after that.

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